The people were being oppressed by a cruel tyrant.
Literal
People [topic-は] cruel tyrant [by-によって] were-being-oppressed.
によって marks the agent of a passive — equivalent to English 'by' — and is more formal than に in this role. 虐げられていた combines the passive 虐げられる with ~ていた to describe an ongoing state in the past: 'were being oppressed' over a period, not a single moment.